Earlier, officials had said they were preparing for a possible filing of charges Sunday but cautioned that the timing was fluid because of Mr. Tsarnaev's medical issues. Officials said the 19-year-old was drifting in and out of consciousness at a hospital after being shot in the throat during a confrontation with authorities early Friday morning. It wasn't clear who had fired the shot.

His brother, Tamerlan Tsarnaev, 26, was the other bomber last Monday, authorities say.

A spokeswoman for the U.S. attorney's office in Massachusetts said there would be "no press conference or further statement" from the office Sunday on the bombing investigation.

Photos: Camaraderie After Boston Chaos

People gathered Sunday on Boylston Street at a memorial for victims of the Boston Marathon bombings. Nicolaus Czarnecki/Zuma Press

Timeline: Boston Bombings and the Search for Suspects

Map: Boston Area

Timeline: Terror in the U.S.

The federal High Value Detainee Interrogation Group, a specialized interrogation team led by the Federal Bureau of Investigation agents, is on standby in case Mr. Tsarnaev becomes capable of communicating, even if only in writing, officials said.

Investigators have alleged that the brothers carried out the Boston Marathon bombings on April 15, which killed three people and injured more than 175. Law-enforcement officials continued investigating crime scenes in and around Boston on Sunday.

Authorities gave no indication as to what motivated the brothers. Their family roots stretch to the Russian republic of Chechnya, which has been a wellspring of terrorism over the years.

One thread has emerged from the FBI's own files. The FBI said that in early 2011, it interviewed the older brother, Tamerlan, at the request of the Russian government but found nothing suspicious. The Russian government had expressed concern that Tamerlan Tsarnaev planned to travel to Russia to engage in possible terrorist activities. Travel records show that he traveled to Russia in January 2012 and returned six months later.

The FBI disclosed in a written statement that the request said Tamerlan Tsarnaev had become a follower of radical Islam "and that he had changed drastically since 2010 as he prepared to leave the United States for travel to the country's region to join unspecified underground groups."

Some friends and family said Tamerlan Tsarnaev had become increasingly religious in recent years. Ruslan Tsarni, an uncle, said Saturday on NBC that the older brother had been radicalized in the U.S. "There is someone who brainwashed him, some new convert to Islam,'' he said. "I was shocked when I heard his words, his phrases, when every other word he starts sticking in words of God."

A day earlier, Mr. Tsarni called his nephews "losers" and said he didn't believe they had an ideological motive.

A developing timeline of the Boston Marathon bombing and subsequent suspect manhunt.

Where did the alleged bombers of the Boston Marathon come from? What were their career aspirations? What can we learn from their online media presence? WSJ's Jason Bellini has "The Short Answer."

ENLARGE

Investigators worked near the location where a suspect in the Boston Marathon bombings was arrested Friday night.
AP

"I think all of the law-enforcement professionals are hoping, for a host of reasons, that the suspect survives because we have a million questions, and those questions need to be answered," Massachusetts Gov. Deval Patrick told reporters outside of Fenway Park, home of Major League Baseball's Red Sox, during an impromptu news conference.

Mr. Patrick said there was still evidence that needed "to be run to ground" and a prosecution to assemble. "There's a lot yet to come." He declined to comment on potential motives of the Tsarnaev brothers.

A federal law-enforcement official familiar with the investigation said Saturday that investigators found five pipe bombs and three grenades after the firefight early Friday morning. The official said investigators were working to determine the origin of the explosives.

"You can go into an Army surplus store and get a hollowed-out shell" of a hand grenade, the official said. A buyer purchasing black powder, likely used in the pipe bombs, isn't required to fill out any paperwork if the purchase is fewer than 50 pounds.

Authorities recovered at least one handgun and one long gun. The official said it wasn't clear yet how the men obtained the firearms. Under federal law, Dzhokhar Tsarnaev would be prohibited from purchasing the handgun but could legally buy a shotgun or rifle.

The manhunt for Mr. Tsarnaev paralyzed the Boston metropolitan area Friday after authorities put much of the area on lockdown, urging residents to stay inside. The end to the weeklong ordeal unfolded after the request was lifted and a Watertown resident alerted police after going to his backyard and finding blood on a tarp-covered boat—and Mr. Tsarnaev hiding inside. Police raced to the yard and engaged in a gunfight with the suspect, surrounding him. FBI agents eventually converged on the vessel and took Mr. Tsarnaev into custody.

Police said it was unclear if Mr. Tsarnaev sustained additional injuries in the shootout at the boat.

"CAPTURED!!! The hunt is over," tweeted the Boston Police Department. "The search is done. The terror is over."

Neighborhood residents cheered police after the arrest. "God Bless America," an officer said over a megaphone as police vehicles ferried away Mr. Tsarnaev. "We've closed an important chapter in this tragedy," President Barack Obama said after the capture was announced.

The arrest capped a violent overnight spree that began with the fatal shooting of a campus police officer at the Massachusetts Institute of Technology and included a police chase through Boston suburbs. The brothers, carrying an arsenal of explosives, fled police before dawn Friday in a stolen Mercedes and hurled pipe bombs in a firefight with police, which left one officer injured.

The older brother, Tamerlan Tsarnaev, wearing what appeared to be an explosive vest, was shot by police and died shortly after, while the younger brother narrowly escaped—after first running over Tamerlan's body with the Mercedes, officials said.

The ethnic Chechens suspected of planting the Boston Marathon bombs have put the spotlight on Chechnya, the embattled Russian republic that's been engaged in fierce fighting for its independence. WSJ's Mark Scheffler reports.

Friends said the two brothers, especially Dzhokhar, had settled comfortably into U.S. society after arriving in the early 2000s. Dzhokhar was enrolled at the University of Massachusetts Dartmouth.

The brothers, who lived in Cambridge, Mass., had escaped notice for three days after Monday's bombings. The younger brother spent time with friends at the college campus, witnesses told investigators.

Late Thursday, the FBI released pictures and videos taken from security cameras near the marathon's finish line. The broad release of the images apparently triggered a chaotic getaway attempt by the brothers.

"Releasing that video really expedited things. People actually did recognize them," said Rep. Michael McCaul (R., Texas), chairman of the House Homeland Security Committee. "That moved things along more quickly, to the point they decided they had to get out of town."

Political fights largely were sidelined during the hunt for the bombers. But on Friday evening, Sen. Lindsey Graham (R., S.C.), via his Twitter account, suggested that the White House consider holding the suspect in military custody. Mr. Obama has ordered that U.S. citizens who are accused of terrorism offenses, such as the younger Mr. Tsarnaev, should face trial in civilian courts.

After Monday's attack, the brothers apparently stayed around the neighborhood where they had settled after immigrating, authorities said.

On Tuesday, Dzhokhar Tsarnaev came to an auto-body shop to pick up a Mercedes he had dropped off for repairs, said Gilberto Junior, who works at the shop.

The younger brother was jittery, said Mr. Junior. "He was biting his nails, and when he was talking to me, his legs were like this," Mr. Junior said, shaking one leg. "He said, 'I need the car now. I need the car right now.' "

At 5:20 p.m. Thursday, the FBI released the photos and video of the brothers. Investigators said the two men apparently decided they needed a car to flee. It wasn't clear why they didn't use the Mercedes from Mr. Junior's shop.

Shortly before 10:20 p.m., MIT police officer Sean Collier, 26 years old, was ambushed and fatally shot in his car. Massachusetts State Police officials at first thought the shooting was unrelated to the marathon bombings.

After the MIT shooting, the brothers allegedly used a gun to steal a Mercedes sport-utility vehicle, briefly kidnapping the driver and telling him they were responsible for the bombings, U.S. officials said. The driver escaped from his captors at a gas station in Cambridge, Mass. His captors then used his ATM card to withdraw cash.

The driver notified police, who spotted the stolen Mercedes and began the chase, with the suspects throwing explosive devices out of their vehicle.

Authorities believed they had trapped the car in a densely populated residential neighborhood of Watertown, but the brothers put up a fight.

Adonis Karageorgis, a 35-year-old dental student, said he saw the sky light up from his apartment. "You could just smell the smoke," he said. "It went high in the sky."

Tamerlan Tsarnaev got out of the car and was shot, authorities said. Richard Donohue Jr., a police officer, was seriously wounded in the firefight.

Seeing his brother shot, Dzhokhar Tsarnaev threw the Mercedes into reverse and drove the car over his brother's body, a law-enforcement official said. Then he drove a short distance away, got out of the car and fled on foot, police said.

On Friday, even as their house-to-house searches failed to locate the suspect, they did find at least one significant blood trail that led many involved in the manhunt to believe the young man was seriously wounded from the shootout and limited in his ability to get far, according to law-enforcement officials involved in the case.

Tamerlan was pronounced dead at a hospital Friday at around 1:30 a.m., and the search for the surviving brother intensified. Hundreds of police officers swarmed the Watertown neighborhood, and helicopters flew overhead.

For most of the day, Boston was virtually shut down as authorities advised residents to stay locked indoors. Public transit, including commuter rail, buses and subways, were halted, and Amtrak closed service to and from the city. Schools and offices were closed.

Chalena Graustuck, 23 years old, who lives a few blocks from the brothers' home, said she felt trapped: "It's just scary. I don't feel safe at all. I want to get on a plane and go far away from Cambridge, but now we're stuck here."

Friday evening, the search narrowed to the house with the boat in Watertown. Irene Santoro, who lives on the street, said police arrived at around 7 p.m. "They started coming down in 10s and 20s and 30s," she said. "It was like something you would see on TV...My heart was palpitating."

The suspect was captured at around 8:40 p.m. "This was truly an absolutely intense investigation,'' said Richard DesLauriers, head of the FBI's Boston office. "As a result of that, justice is being served for the victims of these terrible crimes.''

Authorities said it appeared the brothers were acting alone. The FBI and local police on Friday evening briefly took away for questioning three people in their late teens or early 20s at an apartment complex in New Bedford, Mass., not far from where the younger Mr. Tsarnaev was attending college.

Witnesses said an FBI agent at the scene made comments suggesting authorities were interested in gathering information about the suspects.

Corrections & Amplifications The alleged carjacking victim of the accused Boston Marathon bombers escaped from his captors at a gas station in Cambridge, Mass. His captors then used his ATM card to withdraw cash. A previous version of this article incorrectly reported that the victim was forced to withdraw cash and then was released by his captors. The accompanying maps and chronologies also incorrectly said the victim was released. In addition, the carjacking occurred on Brighton Avenue in Boston. The maps incorrectly indicated it occurred in the area around Third Street in Cambridge. The erroneous information originated in a Cambridge Police Department news release issued on April 19 and in law-enforcement briefings.

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